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Old Jun 16, 2006, 3:28 AM   #1
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While fiddling with filters recently I was wondering why it wouldn't be possible to simulate the effect of a grad.ND filter electronically by sampling half the sensor pixels for a different amount of time/at a different level than the other half. For example if the shutter time is 1/30s then read the top half of the pixels after 1/60s and the bottom after 1/30s to achieve the effect of an ND2 hard grad filter (other variations follow trivially from this). Does anyone know any reason why this couldn't be done?

[Insert feature into the generic FZ40 wishlist :-)].
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Old Jun 16, 2006, 8:34 AM   #2
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One limitation would be that a GND can be oriented in any direction. It is quite common in my experience to rotate it to an odd angle to accommodate the directionality of the light and shadow in the picture. That would probably be awkward to implement electronically.


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Old Jun 16, 2006, 6:00 PM   #3
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I've thought about this as well, but not for that purpose. I'm sure there's a reason a digicam captures the info the way it does, otherwise digicams would be able to capture HDR images in one exposure, and overexposure would never be a problem.
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Old Jun 20, 2006, 12:17 AM   #4
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The sensors generally aren't pixel-addressable, you can only read out a row at a time (which also addresses tclune's comment, the effect would be limited to a fixed horizontal GND, although you could move the position at which the effect occurs). So you can't do an HDR readout directly unless the dynamic-range variations occur in neat horizontal bands :-).
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Old Jun 20, 2006, 12:31 AM   #5
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Oops, just realised that the previous comment about sensor readout is a bit misleading, for CCD sensors the readout is limited by the sensor technology itself, with charge packets being read out of the array sequentially and converted to a voltage outside the sensor array before it's sent off-chip. A CMOS sensor does the basic conversion at the sensor site (signal amplification and A/D is still done outside the sensor array) so in theory it's pixel-addressable, however for manufacturing reasons readout is handled a row at a time. Flash memory does the same thing, basic EEPROM cells are bit-addressable, but for improved performance and density they use block read/write where you need to access an entire block to get to one bit.
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Old Jun 20, 2006, 6:42 AM   #6
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Im sure you could use photoshop to create this effect using quick masks and gradation selections
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Old Jun 22, 2006, 12:29 AM   #7
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Well you can improve a photo in PS with layer gradients and masks -I do it all the time - but that is not why you use ND filters.

Just to darken or colour parts of an image slightly used to be the domain of filters - but digital has rendered most of such filters fairly obselete, since it is better to do these effects in post-process.

However, with scenes of wide dynamic range (common in landscapes with bright skies) the sensor (or film for that matter) can not capture enough stops of info, and you either get burnt highlights (which cannot be recovered in PS) or overdark shadows (whicg end up with too much noise when brightened in PS). This makes the graduated ND still indispensible for the landscape photographer...

You also need to use NDs (sometimes stacked) to get appropriately slowshutter speeds (eg to blur running water), another effect not possible to do suitably in post-process.

I think that a variable-exposure sensor (esp. to address the problem of highlight blowing) is probably theoretically possible in the near future - whether we will see one on a digicam anytime soon is another question...

:?
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Old Jun 22, 2006, 12:49 AM   #8
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in these situations i would take 2 shots at 2 different metering positions and use a gradual blend to merge together to gain correct exposure over all using ps


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Old Jun 22, 2006, 8:24 AM   #9
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There is an useful set of plugins which does something like that.

Many plugins have a double gradient, so that you can apply brightness and colors adjustment on the upper part or on the lower part of the image only, and a nouanced effect is present amid the two parts.

It's DigitalFilmtools "55mm"

see the various filters' effect here:

http://www.digitalfilmtools.com/55mm/index.htm


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Old Jun 22, 2006, 11:32 AM   #10
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David C said, "I think that a variable-exposure sensor (esp. to address the problem of highlight blowing) is probably theoretically possible in the near future - whether we will see one on a digicam anytime soon is another question... "

David, check out the upcoming Sony A100 DSLR. It's going to have a feature called the "Dynamic Ranger Optimizer" which can apparently work to keep both dark and bright areas well exposed. I haven't seen a good technical description of how this works, but it may be the best solution yet. In fact, the camera as a whole looks quite appealing with the incredible feature set and relatively cheap price.
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